The objective optics used in digital cameras are typically designed so as to minimize the optical point spread function (PSF) and maximize the modulation transfer function (MTF), subject to the limitations of size, cost, aperture size, and other factors imposed by the camera manufacturer. The PSF of the resulting optical system may still vary from the ideal due to focal variations and aberrations. A number of methods are known in the art for compensating for such PSF deviations by digital image processing. For example, U.S. Pat. No. 6,154,574, whose disclosure is incorporated herein by reference, describes a method for digitally focusing an out-of-focus image in an image processing system. A mean step response is obtained by dividing a defocused image into sub-images, and calculating step responses with respect to the edge direction in each sub-image. The mean step response is used in calculating PSF coefficients, which are applied in turn to determine an image restoration transfer function. An in-focus image is obtained by multiplying this function by the out-of-focus image in the frequency domain.
PCT International Publication WO 2004/063989 A2, whose disclosure is incorporated herein by reference, describes an electronic imaging camera, comprising an image sensing array and an image processor, which applies a deblurring function—typically in the form of a deconvolution filter (DCF)—to the signal output by the array in order to generate an output image with reduced blur. This blur reduction makes it possible to design and use camera optics with a poor inherent PSF, while restoring the electronic image generated by the sensing array to give an acceptable output image.
Low-cost color video cameras typically use a single solid-state image sensor with a multi-colored mosaic filter overlay. A mosaic filter is a mask of miniature color filter elements in which a filter element is positioned in front of each detector element of the image sensor. For example, U.S. Pat. No. 4,697,208, whose disclosure is incorporated herein by reference, describes a color image pickup device that has a solid-state image sensing element and a complementary color type mosaic filter. Any sort of image sensor with a color mosaic filter, regardless of the choice and arrangement of the colors in the mosaic, is referred to hereinbelow as a “mosaic image sensor.”
The filter elements in the mosaic filter generally alternate between the primary RGB colors, or between the complementary colors cyan, magenta and yellow. One common type of color mosaic filter is called a “Bayer sensor” or “Bayer mosaic,” which has the following general form (in which letters represent colors—R denotes red, G denotes green and B denotes blue):
RGRGRGGBGBGBRGRGRGGBGBGBRGRGRGGBGBGBThe different color filters have respective passbands, which may overlap. The Bayer mosaic is described in U.S. Pat. No. 3,971,065, whose disclosure is incorporated herein by reference.
Processing the image produced by a mosaic image sensor typically involves reconstructing the full color image by extracting three color signals (red, green and blue) from the sensor output. An image signal processor (ISP) processes the image sensor output in order to compute luminance (Y) and chrominance (C) values for each pixel of the output image. The ISP then outputs these values (or the corresponding R, G and B color values) in a standard video format.